Production of the Light p-Process Nuclei in Neutrino-driven Winds

Recent studies of nucleosynthesis that occurs in the neutrino-driven wind following the delayed explosion of a Type II or Ib supernova have provided a promising site for the synthesis of the r-process isotope& One worrisome aspect of these models is a large overproduction of N = 50 (closed neutron shell) nuclei in about 0.01 Msun of the ejecta with entropy per baryon S/(NAk) ≍ 50 originating before the r-process epoch, at times t ≲ 1 s after core bounce. This overproduction is a consequence of the low electron fraction (Ye ≍ 0.46) employed in these calculations. We have found that raising ye in the wind at this time to ≍0.485 cures the difficulty. The problematic nucleosynthesis disappears and is replaced by the moderate production of )⁷⁰Ge and some interesting light p-process nuclei, )⁷⁴Se, )⁷⁸Kr, )⁸⁴Sr, and )⁹²Mo, thus potentially turning a major shortcoming of this model into a success. The early neutrino- heated ejecta must have Ye ≳ 0.484 to effect a cure of the N = 50 overproduction problem and produce the light p-nuclei up to )⁹²Mo. If Ye ≳ 0.488, production of light p-nuclei is lost, but the N = 50 overproduction is still avoided. In addition, )⁶⁴Zn is produced in amounts that could be important for galactic chemical evolution. It is interesting that the r-process and some light p-process nuclei may be coproduced. We are also able to place approximate limits on the mass a typical Type II supernova can eject as a function of Ye. In particular, the ejected mass having Ye ≲ 0.47 must be ≲ 10⁻⁴ Msun.